Melanosomes are a specialized organelle of melanocytes that exist in order to conduct a highly specialized task, producing and storing melanin pigment. Melanin acts as a protective agent against the damaging effects of UV radiation. Thus, the presence of melanin in human skin normally contributes to a key element promoting health. Unfortunately, melanosomes are also associated with disease causing potential. Of key concern, many of the intermediates of melanin synthesis are highly reactive quinones and capable of causing significant oxidative damage. Aberrant oxidative stress associated with pigment production has specifically been implicated in several skin diseases, including cutaneous autoimmune disease and malignant melanoma. Despite this importance, the biological processes and ultrastructural components contributing to the ability of melanosomes to contain or detoxify these compounds are largely unknown. Here, we present our preliminary experiments identifying a new melanosomal protein, GPNMB, which we hypothesize participates in the ability of melanosomes to contain or detoxify the potentially cytotoxic intermediates of melanin synthesis. We will test our hypothesis using a combination of genetic epistasis experiments that take advantage of existing Gpnmb mutant mouse strains and synchrotron based imaging techniques that utilize scanning transmission X-ray microscopy. Completion of these experiments will identify a function of the GPNMB protein in melanosomes of the skin, and begin to clarify the role of this protein in health and diseases of pigmentation. Melanosomes are a specialized organelle of melanocytes that in health produce and store melanin pigment and in disease contribute to a variety of skin diseases involving melanocytes. The goal of this research is to initiate functional tests of the newly identified melanosomal protein, GPNMB, utilizing a combination of genetic and innovative imaging approaches. Completion of these studies will yield important gains concerning the melanosomal functions of GPNMB and create needed resources that will facilitate growth of this area of research.